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UNDERSTANDING
HEREDITY
Part 1
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THE WORK OF GREGOR
MENDEL
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The Work of Gregor Mendel
• ALL living things have a set of
characteristics that are contained in
genes.
• These genes come from our parents
and are found in every cell in our
body.
• Genetics – the scientific study of
heredity
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The Work of Gregor Mendel, cont.
• Gregor Mendel – an Austrian Monk
born in 1822
• He laid the foundation for much of
our understanding of inheritance
patterns
• Credited as the “Father of Genetics”
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The Work of Gregor Mendel, cont.
• He was a mathematician/botanist
and was in charge of the monastery
garden
• He noticed that individual plants of
the same species were not identical
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The Work of Gregor Mendel, cont.
• He wanted to know
why they were not
identical, so he
experimented on
pea plants to help
answer the
question.
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The Work of Gregor Mendel, cont.
• He selected
seven traits
found in pea
plants to study:
1.
2.
3.
4.
5.
6.
7.
Seed shape
Seed color
Pod shape
Pod color
Plant height
Flower color
Flower position
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The Work of Gregor Mendel, cont.
Why pea plants?
1. Pea plants are pure breeding –
they produce identical offspring
when they self-pollinate
2. They grow fast
3. They have traits in distinct
alternate forms (either/or)
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The Work of Gregor Mendel, cont.
• These characteristics allowed
Mendel to control the outcome when
he cross-pollinated plants with
contrasting traits
• The resulting offspring are called a
monohybrid cross
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The Work of Gregor Mendel, cont.
How did he make these monohybrid
crosses?
1. Mendel prevented self-pollination
in the plants by removing the
stamen
2. He dusted the pollen from one
stamen onto another plant’s pistil
(cross polination)
3. The result: cross-breed plants
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The Work of Gregor Mendel, cont.
What happened next?
• He called the original plants the
Parent (P) generation
• The offspring produced by the P
generation were the F1 generation;
also called hybrids
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The Work of Gregor Mendel, cont.
• All of the hybrids showed the traits
of only one of their parents…
• The traits from the other parent
had disappeared!
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The Work of Gregor Mendel, cont.
Where did those traits go?
• To answer that question, Mendel let the
F1 plants self-pollinate
• This produced the F2 generation:
~ ¾ of the plants showed the traits of
their parents (the F1 generation)
~ ¼ of the plants showed the traits of
their grandparents (the P generation)
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The Work of Gregor Mendel, cont.
• This lead Mendel to make two
conclusions about what he called
biological inheritance (we call it genes):
1. Traits are passed from one generation
to the next
2. Each trait is found in at least 2
contrasting forms
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The Work of Gregor Mendel, cont.
• He further concluded that:
a. Traits are inherited as distinct units from
the parent
b. Organisms inherit 2 copies of each unit (one
per parent)
c. Organisms donate one of those copies when
they make gametes
d. The 2 copies separate (segregate) during
gamete formation
• These conclusions became know as the Law of
Segregation
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MODERN GENETICS
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Modern Genetics
• Some traits are
dominant over
other traits
• The unit that
seems to disappear
is recessive – it
can only be
expressed when 2
recessive traits
combine
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Modern Genetics, cont.
• We refer to traits as genes
• Genes are sections of chromosomes
• Each form of the gene is called an allele
• An organism can be:
Homozygous – having 2 identical alleles
OR
Heterozygous – having 2 different alleles
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Modern Genetics, cont.
• Phenotype – the physical characteristics
of the organism (what it looks like)
• Genotype – the genetic makeup of the
organism (what is actually there)
The phenotype of an organism is the
result of:
1. The Genotype
2. Environmental pressures
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PROBABILITY AND
PUNNETT
SQUARES
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Probabilities
• Mendel realized that the Principle of
Probability (the likeliness that a particular
event will occur) could be used to predict and
explain the results of genetic crosses.
• If there are 2 possible outcomes, then there
is a 1 in 2 or 50% chance of each outcome
occurring.
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Probabilities, cont.
• Example: If you flip a
coin 3 times in a row,
what are the chances
it will be heads up
every time?
½ x ½ x ½ = 1/8
• Probabilities can
predict the average
outcome of a large
number of events –
not the outcome of an
individual event.
• For that, we need
Punnett Squares…
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Punnett Squares
• The gene combination that might result
from a genetic cross can be predicted
and compared with a Punnett Square
• The dominant allele is represented by a
capital letter (like T for tall)
• The recessive allele is represented by a
lower case letter for the same trait (like
t for short)
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Punnett Squares, cont.
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Punnett Squares, cont.
• Monohybrid cross - cross involving a
single trait
ex. flower color
• Dihybrid cross - cross involving two
traits
ex. flower color & plant height
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NON-MENDELIAN
GENETICS
Beyond Dominant &
Recessive Alleles
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Beyond Dominant & Recessive Alleles
• Principle of Independent Assortment: genes for
different traits can segregate independently
during the formation of gametes
• This accounts for the genetic variations among
organisms of the same species!
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Beyond Dominant & Recessive Alleles, cont.
• The majority of genes have more than
two alleles
• Many traits are controlled by more than
one gene
• Most of the genes that affect the
physical appearance of an organism are
found on the autosomes
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Beyond Dominant & Recessive Alleles, cont.
1. Incomplete dominance – case where one allele
is not completely dominant over another;
produces an intermediate type
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Beyond Dominant & Recessive Alleles, cont.
2. Codominance – both alleles contribute
to the phenotype; it is a blend of the two
alleles
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Beyond Dominant & Recessive Alleles, cont.
3. Multiple Alleles – one individual can only
have two alleles but more than two alleles can
exist in a population.
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Beyond Dominant & Recessive Alleles, cont.
4. Polygenic traits – many traits are
produced by the interaction of several
genes
Examples: hair, eye and skin color
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Beyond Dominant & Recessive Alleles, cont.
5. The characteristics of an organism are also
determined by the environment it lives in